Update mean_en.i test to run with DensityMoles instead of Density.
Update mean_en.i test to run with DensityMoles instead of Density.

shannon-lab/zapdos,lindsayad/zapdos,lindsayad/zapdos,jhaase1/zapdos,jhaase1/zapdos,shannon-lab/zapdos,jhaase1/zapdos,shannon-lab/zapdos,lindsayad/zapdos